The present invention relates in general to lamps based on light-emitting diodes (LEDs) and in particular to procedures for tuning the color of light produced by lamps that include multiple LEDs.
With the incandescent light bulb producing more heat than light, the world is eager for more efficient sources of artificial light. LEDs are a promising technology and are already widely deployed for specific purposes, such as traffic signals and flashlights. However, the development of LED-based lamps for general illumination has run into various difficulties. Among these is the difficulty of mass-producing lamps that provide a consistent color temperature.
As is known in the art, not all white light is the same. The quality of white light can be characterized by a color temperature, which ranges from the warm (slightly reddish or yellowish) glow of standard tungsten-filament light bulbs to the cool (bluish) starkness of fluorescent lights. Given existing processes for LED manufacture, mass-producing white LEDs with a consistent color temperature has proven to be a challenge.
Various solutions have been tried. For example, white LEDs can be binned according to color temperature and the LEDs for a particular lamp can be selected from the desired bin. However, the human eye is sensitive enough to color-temperature variation that a large number of bins is required, with the yield in any particular bin being relatively low.
Another solution relies on mixing different colors of light to produce a desired temperature. For example, an LED lamp can include a number of white LEDs plus some red LEDs. The brightness of the red LEDs can be increased to warm the light to the desired color temperature. Such lamps generally require an active feedback mechanism to maintain the color temperature, in part because the LEDs used are not stable in their color characteristics over time. The active feedback mechanism requires a sensor to detect the light being produced, an analyzer to determine whether the light is at the desired color, and an adjustment mechanism to adjust the relative brightness of the white and red LEDs as needed to maintain the desired color. These feedback-loop elements can be a weak point in the system; for example, if the light sensor drifts over time (as most do), so will the color of the light. In addition, incorporating active feedback components into a lamp drives up the cost of manufacturing (and operating) the lamp.